Due to the explosive growth in the popularity and ubiquity of the Internet and World Wide Web in recent years, and motivated by huge potential business opportunities, the online advertising market continues to grow at a rapid pace. Correspondingly, online systems are experiencing a continually increasing volume of users who perform various Internet activities such as online information searches and webpage visits. Internet search engines generally allow advertisers to deliver relevant advertisements to users (i.e., prospective consumers) using online advertisement delivery systems. A given advertisement delivery system attempts to make the advertisements it delivers to a given user relevant to the user by taking into account the characteristics of the content of the online information that is delivered to the user, and also taking into account the characteristics of the user that may be known to or inferred by the advertisement delivery system.